The present invention relates to methods and devices which make it possible to heat-treat metal wires, particularly carbon-steel wires, for instance for the purpose of obtaining a fine pearlitic structure. These wires are used, in particular, to reinforce rubber and/or plastic articles, for instance automobile tires.
French Patent Application 88/00904 describes a method and a device for effecting a pearlitization treatment in which the wire is passed within one or more tubes containing a gas which is practically without forced ventilation. That method and device have the following advantages:
simplicity, low investment and operating expenses;
a precise law of cooling can be obtained and recalescence avoided;
a pearlitization treatment can be carried out with the same installation on wires whose diameter varies within wide limits;
all problems of hygiene are avoided and cleaning of the wire is not necessary since the use of molten metals or salts is avoided.
Experience shows, however, that for steels of a slightly different chemical composition (in particular carbon content slightly below or above the eutectoid), the TTT (time, temperature, texture) curves may be very different. This phenomenon is even observed in the case of steels which have identical chemical compositions but come from different steel mills.
Thus, by way of example, in the case of steels with 0.8% carbon it is current practice to have incubation times which vary within ratios of 1 to 1.7, the incubation time being the time elapsing between the start of the cooling and the start of the austenite/pearlite transformation, this making it necessary to use installations having different construction parameters in order to treat steel wires of the same diameter and identical or similar compositions in order to obtain an optimal steel structure in all cases.